The nitrogenase complex is extremely sensitive to inactivation by O2, so the enzyme must be protected from this reactive substance. During the root nodules of leguminous plants, protection is afforded by the symbiotic synthesis of leghemoglobin. The globin part of this monomeric oxygen-binding protein is synthesized by the plant, whereas the heme group is synthesized by the Rhizobium. Leghemoglobin has a very high affinity for O2 for maintaining a low enough concentration to protect the nitrogenase.